In general, an automotive vehicle is provided with a hydraulic power transmission such as a torque converter between an engine and a transmission mechanism forming part of an automatic transmission (hereinafter simply referred to as an automatic transmission). There have so far been developed a wide variety of hydraulic power transmissions one of which is operative to take a power transmitting state in which the hydraulic power can be transmitted from the engine to the input shaft of the automatic transmission (hereinafter simply referred to as a transmission input shaft) while the rotation difference between the engine and the automatic transmission input shaft is being absorbed at the starting time of the engine and other timings. The hydraulic power transmission operated always only in the hydraulic power transmission state results in decreasing a power transmission efficiency. The hydraulic power transmissions are each generally provided with a lock-up mechanism which is operative to take a lock-up state in which the power from the engine can be transmitted directly to the transmission input shaft without being transmitted through hydraulic power transmission apparatus in a specified drive state such as a high speed travelling state and the like.
The lock-up mechanism is constituted by a lock-up clutch having at least one friction engagement surface between a front cover connected with a rotation member of the engine and a lock-up piston connected with a rotation member of the transmission input shaft through a damper mechanism. The lock-up mechanism is adapted to take the lock-up state when the lock-up clutch is completely being engaged.
For this reason, there has in recent years been widely used a lock-up mechanism which is operative to execute a slip control allowing slips to be caused to the lock-up clutch, thereby expanding a wider driving area, where the power transmission can be achieved with a higher efficiency than only hydraulic power transmission, to a lower speed traveling state or at the transmission state than in the conventional driving area to improve fuel consumption.
The lock-up clutch of this kind is constituted for example by a multiple of plates having three friction surfaces or more in order to increase the heat capacity of the plates and thus suppress the temperature of the friction plates from being raised (see for example Patent Document 1).
Further, another known mechanism (see for example Patent Document 2) comprises a plurality of clutch discs having respective clutch facings (i.e., friction materials) mounted thereon. Each of the clutch facings has an outer peripheral portion formed with annular circumferential grooves and communication grooves each having one end held in communication with the annular circumferential groove and the other end open at the inner peripheral portion of the facing, thereby suppressing a drag torque from being caused at the lock-up release time.
Furthermore, still another known lock-up mechanism (see for example Patent Document 3) is constructed to have substantially annular circumferential grooves formed on friction surfaces of friction materials or engagement surfaces of a front cover or a lock-up piston to prevent shudders facilitated to be caused in the slip control state (shudders mean vibrations of the entire vehicle caused stemming from a stick-slip phenomenon and the like of the friction engagement portions forming parts of the lock-up clutch).